Q: ios9.0.2 not compatible with itunes 12.1.2?

Hello minimumsix, I'm sorry that you didn't get an answer sooner; the iTunes for Windows community probably would've been the best place for this question. You are correct that iTunes 12.1.2 does not support iOS 9, and that Vista cannot be updated to iTunes 12.3. But the good news is that Apple released special version 12.1.3 that supports both Vista and iOS 9: Apple - Support - Downloads. I would uninstall 12.1.2 first.

Your library is not deleted when you uninstall the software, but you can further back it up if you wish. I'm not certain that uninstalling 12.1.2 is necessary, but that's the way I did it (I am running 32-bit Vista myself). I followed turingtest2's uninstalling instructions in Troubleshooting issues with iTunes for Windows updates.